The present invention relates to a metal cutting insert which is primarily intended for turning operations, the insert being indexable, i.e., it has more than one operative corner cutting edge and so that when one corner has been worn out, the insert can be turned or indexed so that a new corner becomes active.
For all sorts of turning operations, there is a certain interplay between the feed rate and the corner radius, the corner radius being the connection between the main cutting edge and the secondary cutting edge. Thus, for rough machining, a largest possible corner radius should be chosen in order to obtain a strong cutting edge. On the other hand, a corner radius too large may cause vibrations. Therefore, for fine turning usually a smaller corner radius is selected, normally smaller than 2 mm. The disadvantage with a small corner radius is that it increases the wear rate of the insert, thereby deteriorating its life and performance. The surface smoothness produced on the workpiece is specifically influenced by the interplay between the corner radius and the feed rate.
The entering angle is defined as the angle between the main cutting edge and the direction of feed. This angle has a considerable influence on the interrelation between the different cutting force components, and thereby also on the surface smoothness and dimension accuracy. The angle of tool back clearance is the angle between the secondary cutting edge and the direction of feed and it affects the smoothness of the turned surface. It is evident that for a specific insert it is not possible to alter the entering angle without simultaneously changing the angle of tool back clearance. Thus, surface smoothness and dimension accuracy are very sensitive to changes of the entering angle.
A fundamental problem for all turning operations is that the desired smoothness is not achieved. Sometimes the surface smoothness can be improved by using higher cutting speeds and neutral or positive rake angles. However, the sensitivity to the adjustment of the entering angle remains.
Thus, a first object of the present invention is to improve the smoothness of the machined surface.
A second object of the present invention is to reduce the sensitivity of a turning cutting insert relative to the positioned entering angle.